Fabrication and characterisation of a Nb diffusion-cooled hot electron bolometer for a 730 GHz waveguide mixer

We have fabricated Nb diffusion-cooled hot electron bolometers for a waveguide mixer around 700 GHz. The device in this work is a thin (12 nm) Nb bridge with a length and width of 220 nm and is defined by a two-step electron beam lithography process. The resistance as a function of temperature shows two transitions, which we attribute to the Nb bridge (6.1 K) and to parts of the Nb which are covered by the Au pads (5.4 K). We have measured a DSB receiver noise temperature of 2200 K at 735 GHz and a bath temperature of 3.3 K. No significant increase of the noise temperature is observed at IF frequencies between 1.1 and 1.7 GHz. The coupled LO power is estimated to be 35 nW